Visibility in Weather Science: Meaning, Measurement Methods and IMD Classification

Context

Recent episodes of dense fog and smog across North India have highlighted the importance of scientifically measuring and classifying visibility in meteorology, especially for aviation safety and surface transport.

What is Visibility?

In meteorology, Visibility is expressed in terms of Meteorological Optical Range (MOR).

Definition:
MOR is the distance over which a beam of light passing through the atmosphere is reduced to 5% of its original intensity due to scattering and absorption by atmospheric particles.

In simple words, it is the maximum distance at which an object can be clearly seen under given atmospheric conditions.

Factors Affecting Visibility

Visibility is reduced by the presence of:

• Fog and mist (water droplets)

• Dust and sand

• Smoke

• Aerosols

• Pollution and particulate matter (PM2.5, PM10)

• Smog (mixture of smoke + fog)

Higher the concentration of suspended particles → lower the visibility.

Instrument-Based Measurement of Visibility

Modern meteorology uses automated instruments instead of visual observation.

1. Direct Method – Transmissometer

• A transmitter sends a laser/light beam to a receiver.

• Distance between them: usually 20–75 metres.

• The receiver measures how much light reaches it.

• Loss of intensity is caused by:

  • Scattering

  • Absorption by particles

• Using mathematical formulas, this loss is converted into visibility distance (MOR).

Used in:

• Airports

• Runways

• High-precision weather stations

Advantage: Very accurate
Limitation: Expensive and needs maintenance

2. Indirect Method – Scatterometer / Forward Scatter Sensor

• The light beam is not aimed directly at the receiver.

• In clear air, almost no light reaches the receiver.

• When fog, dust, or smog is present:

  • Particles scatter light

  • Some scattered light reaches the receiver

• The amount of scattered light is proportional to particle concentration.

• This is converted into visibility.

Used in:

• Automatic Weather Stations (AWS)

• Road weather monitoring systems

Advantage: Compact, economical
Limitation: Slightly less accurate than transmissometer

Visibility Classification in India (IMD Standards)

The India Meteorological Department (IMD) classifies visibility mainly for fog conditions:

Importance of Visibility Classification

Critical for:

1. Aviation

   • Aircraft landing and take-off safety

   • Instrument Landing System (ILS) operations

2. Road Transport

   • Accident prevention

   • Highway advisories

3. Railways

   • Train speed regulation

   • Fog safety devices

4. Disaster Management

   • Early warning systems

   • Emergency planning

5. Urban Pollution Control

   • Smog episodes indicate severe air quality degradation

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